The present invention relates to a device for activating knitting cams in circular knitting machines.
As known, in knitting machines and in particular in machines for knitting socks and stockings, various kinds of knitting are performed by varying the needles which knit and by varying the path followed by the heels of said needles inside the cam boxes of the needle cylinder. In order to obtain these variations, movable knitting cams can be inserted or extracted, according to the requirements, in the cam boxes of the needle cylinder. Said knitting cams can generally have two insertion positions, and some needles are provided with long heels while others have short heels so that by varying the insertion position of the movable cams it is possible to move to knit only the needles with long heels or all the needles.
The movable knitting cams are generally inserted or extracted from the cam boxes of the needle cylinder with an approaching or spacing-apart motion in a radial direction with respect to the needle cylinder by means of levers which are actuated according to the various knitting requirements.
The U.S. Pat. application No. 07/245.248 in the name of the same Assignee, filed on Sept. 16, 1988, which is incorporated herein by reference, discloses a knitting cam activation device which comprises a frame which slidably supports, along a radial direction with respect to the needle cylinder, a slider arranged facing said needle cylinder with one of its ends to which a knitting cam is fixed. Said slider is fixed, at its other end, to the output shaft of a linear motor driven by an electronic control element which supervises the various operations of the machine and, according to the knitting to be performed, activates the linear motor, which causes a preset movement of the knitting cam in a radial direction with respect to the needle cylinder, so as to cause its insertion in, or extraction from, the cam boxes of the needle cylinder.
Said device allows to insert or extract the movable knitting cams in very short times and can therefore be used with excellent results even on machines with a high-speed needle cylinder rotation without necessarily entailing a reduction in the machine operating speed during the movement of the knitting cams.
The movements imparted to the slider by the linear motor are detected by a linear transducer and are compared to reference values by means of the electronic control element of the machine, thereby ensuring a high degree of precision and repeatability in the actuation of the knitting cams.
In the above described patent application, the slider is supported by the frame by means of a plurality of balls which are partially accommodated in longitudinal grooves defined on the outer surface of the slider and rest on the inner surface of a sleeve which embraces the slider for most of its longitudinal extension.
During testing, said knitting cam activation device has been found to be susceptible to improvements, in particular regarding the sliding coupling between the supporting frame and the slider.
In the device described in the patent application No. 07/245.248, in order to obtain a high-precision actuation of the knitting cams it is in fact necessary to safely exclude any rotation, even a partial one, of the substantially cylindrical slider about its axis. In order to obtain a rotation-preventing coupling of the slider, said slider is inserted with the balls in the sleeve so that the balls are loaded in a transverse direction with respect to the axis of the slider. This fact causes a slight radial deformation of the sleeve in the regions occupied by the balls. This deformation, despite being minimal, can complicate the insertion of the sleeve in the cylindrical seat appropriately provided in the supporting frame. After assembly, the forces which act on the balls and ensure a rotation-preventing coupling to the slider can negatively affect the operation of the linear motor, which must overcome considerable friction resistances when activating the slider.